1. Field of the Invention
The present invention relates to the field of radio frequency identification (RFID) and, more particularly, to a radio frequency identification reader antenna and a shelf.
2. Description of the Related Art
Radio frequency identification (RFID) technology has been widely used recently, and smart shelves are an application scenario of the RFID system. In many shelf applications, the following three questions are particularly important: (i) what kind of goods are on the shelf, (ii) the location of the goods on the shelf, and (iii) when the goods are placed on the shelf or removed from the shelf. Before using RFID technology, the answer to these three questions needs to be obtained relying on manual operations of the staff. For example, the staff have to use a device, such as a barcode scanner, to scan the goods manually. However, after the use of RFID technology, the answer to these three questions can be automatically obtained. The RFID reader can easily determine the tags affixed to the goods, enabling the system to answer these three questions according to the information read from the tags.
FIG. 1 shows an application scenario of a smart shelf with an RFID function. In the application shown in FIG. 1, empty boxes 101 with tags are placed on the top layer 104 of the shelf. At this moment, the reader must be able to read the boxes that are placed in any location on the top layer 104, and prevent two possible cross readings, i.e., reading of lower-layer boxes 102 with tags (such as a box loaded with metal), and reading of boxes that are carried by a person before being placed on the shelf. It should be noted that each layer of the shelf has a metal bottom.
In the prior art, the high frequency (HF) RFID solution is widely used in smart shelves as shown in FIG. 1. It is not difficult to design an HF coil with a size of the antenna 103 as shown in FIG. 1, but the HF RFID has its own problems. On the one hand, the tags of HF RFID are expensive. On the other hand, it is difficult to obtain a far-field/near-field (FF/NF) hybrid solution, i.e., it is impossible to satisfy the need of smart shelves and far-field reading (such as warehousing entrance and exit management) at the same time using only one tag.
Compared with HF RFID, near-field (NF) Ultra High Frequency (UHF) RFID is a more promising technology. Existing NF UHF RFID can use small and cheap tags to obtain a controllable reading area, and it is easy to design an FF/NF hybrid solution. However, due to the difficulty in designing a large planar antenna, this type of antenna can now only be used in narrower shelves, such as book shelves. In practical use, this antenna can be made very long, but it is difficult to make it wide.
For example, the Coplanar Waveguide (CPW) antenna, product model HR RFD-NF09, developed by Hiray Microwave Science and Technology Co., Ltd. is such an NF UHF RFID narrow antenna. For another example, an article published by Carla, et al in IEEE Antennas and Propagation Magazine, vol. 7 in 2008, entitled “RFID Smart Shelf with Confined Detection Volume at UHF” proposed a leaking microstrip line for book shelves.
In order to use a narrow antenna on a large shelf as shown in FIG. 1, the only option is to place the antenna on the edge of the top layer. When the staff lift up the box by the edge of the shelf, the reader will read the tag. However, this solution has the following problems:
1. The antenna must be very narrow (less than 2 cm) to avoid field null reading.
2. Although this type of antenna can read tags which it passes by, its narrower width will affect the reading time. Taking a width of 2 cm and a pass-by speed of 2 m/s, for example, the reading time available for the reader to use is only 10 ms. This will greatly restrict the situation where multiple shelves share one reader.
In order to obtain a planar antenna having a larger size, yet another possible solution is to use leakage slot designs. The article published by Wonkyu Choi, et al in IEEE APSURSI in 2009, entitled “UHF Near-Field Antenna for RFID Smart Shelf” proposed a slot array fed by a microstrip line as shown in FIG. 2. Specifically, this design proposed an electrical coupling planar antenna based on slots, and the basic idea is centered on the use of a microstrip line to feed the slot array on the ground plane, where the slot is at the current peak of the microstrip line. The advantage of this design is that the antenna can be adjusted for shelves of different sizes. However, the field null reading problem of this design is particularly significant, and electrical coupling causes cross reading problems easily. Moreover, in order to construct an array, a power divider may reduce the current fed to each slot.
Therefore, all currently designed UHF RFID antennas have certain problems and cannot be well adapted to the requirements for diversification of shelf applications.